1. Field of the Invention
The present invention relates generally to the field of silicone resins and more particularly to such resins having silicone rubbers incorporated into their structure. The rubber-modified silicone resins show a marked improvement in fracture toughness.
2. Description of the Prior Art
For some time, it has been known that making rubber additions to organic polymer resins can increase the toughness thereof. Such systems are disclosed in, for instance, an article entitled Effects of Rubber Additions On The Fracture Toughness Of A Polyester Resin (Tetlow, P. D. et al. Proceedings of the Annual Technical Conference, 1979, Reinforced Plastics/Composites Institute The Society of the Plastics Industry, Inc. Vol. 34, 23F) and a paper entitled Crack Toughened Polyester Resin Formulations (McGarry, F. J. et al., American Chemical Society Division of Organic Coating and Plastics Chemistry Vol. 28, No. 1, pp 526-36) presented in San Francisco in April 1968.
Increasing the toughness of various silicone compositions has also been previously carried out. Maguire et al., U.S. Pat. No. 5,034,061 discloses a silicone resin/fluid polymer adapted to form a transparent, shatter-resistant coating. The composition includes a silicone resin copolymer consisting essentially of R.sub.3 SiO.sub.1/2 and SiO.sub.4/2 units with unsaturated olefinic functional R groups, a polydiorganosiloxane fluid with vinyl functionality, an organopolysiloxane crosslinker having hydrogen functionality and a catalyst. The composition is disclosed as being particularly adapted for use in coating incandescent glass lamps.
Canadian Patent No 691,206 (1964,) Fenner, assigned to Dow Coming Corporation, discloses the use of silica-filled silicone resin/fluid combinations for damping vibrations. The ability of the disclosed silicone resin/fluid compositions to dampen vibrations is illustrated through the measurement of the ratio of G', the elastic shear modulus, to G", the loss shear modulus. The magnitude of this ratio is indicated as being inversely proportional to the ability of the material to absorb vibration. The ratio of G'/G" of the subject materials is compared to that of compositions prepared without a resin constituent.
The above-described toughened silicone compositions are generally of the types having a fairly low modulus of elasticity. Heretofore, successfully increasing the fracture toughness of rigid silicone resins has not been accomplished. As used herein to describe silicone resins, the term "rigid" means that the resin material, in its unfilled condition, exhibits a certain "stiffness" characterized by having a Young's modulus of at least 6.9.times.10.sup.8 Pa. As used herein, the term "unfilled" means that no reinforcing fillers, such as carbon or glass fibers or silica powders have been added to the resin.
Rigid silicone resins have long been employed in applications that take advantage of their heat- and fire-resistant properties. These properties make the silicone resins attractive for use in fiber-reinforced composites for electrical laminates, structural use in automotive components, aircraft and naval vessels. The unmodified rigid silicone resins of the prior art are, however, quite brittle which severely limits their use.
Thus, there exists a need for rigid silicone resins having substantially increased fracture toughness which would permit the exploitation of the unique fire-, electrical- and heat-resistant properties in applications where brittle failure must be avoided.